The accelerators used in the vulcanization of unsaturated polymers, polyolefins or rubbers, in particular halogenated rubbers, such as polychloroprene, are generally thiourea-based accelerators which today compose the crosslinking systems for rubbers having the best performance.
Ethylene thiourea (ETU) is the reference thiourea used today for the vulcanization of chlorinated rubbers, in particular polychloroprene. However, ETU is toxic in itself, classified in carcinogenic group 2, according to the classification of the European Union, the IARC (International Agency for Research on Cancer) and the EPA (Environmental Protection Agency).
It is consequently necessary to henceforth look for replacement products which are less toxic, which are more environmentally friendly and which exhibit an effectiveness at least as good as that of thiourea-based accelerators, in particular ETU, in vulcanization processes.
The literature already provides some examples of vulcanization agents or vulcanization accelerators other than thiourea derivatives. For example, U.S. Pat. No. 4,288,576 describes the use of 2,5-dimercapto-1,3,4-thiadiazole for the vulcanization of saturated chlorinated polymers, in the presence of a basic compound chosen from amines, amine salts, quaternary ammonium salts, aromatic guanidines and the condensation products of aniline with an aldehyde.
As other examples, U.S. Pat. No. 5,391,621 describes the use of certain organopolysulphide compounds derived from 1,3,4-thiadiazole as agents for the vulcanization of chlorinated polymers. Other polysulphides, poly[2,5-bis(polysulphano) -1,3,4-thiadiazoles], are described in U.S. Pat. No. 5,563,240, where they are of use as agents for the vulcanization of polychloroprene.
Patent Application EP 0 796 890 describes the vulcanization of halogenated acrylic rubbers in the presence of 2,5-dimercapto-1,3,4-thiadiazole or derivatives and of a metal dialkyldithiocarbamate.
More recently, Patent Application US 2003/153652 also presents compositions for vulcanizing chlorinated polymers, the said compositions resulting from the mixing of a zeolite compound with a vulcanization agent chosen from mercaptotriazines, thiadiazoles and thiurams.
The publication “An improved curing system for chlorine-containing polymers”, by R. F. Ohm and T. C. Taylor (which appeared in “Rubber World”, March 1997, pages 33 to 38), presents a comparative study between ETU and DMTD (2,5-dimercapto-1,3,4-thiadiazole), which are used as agents for the vulcanization of polychloroprene in combination with vulcanization activators.
Despite these solutions already put forward for the replacement of ETU, a need remains, however, for vulcanization compositions which are even more effective, in particular for the vulcanization of unsaturated polymers, especially of unsaturated halogenated polymers and more particularly of unsaturated chlorinated polymers, more environmentally friendly and capable of conferring, on the vulcanized polymers, mechanical properties and properties of resistance to ageing which are further improved.